Effects of Food Supplementation on Female Nest Attentiveness and Incubation Mate Feeding in Two Sympatric Wren Species

—We examined effects of incubation mate feeding on female incubation behavior and correlates of fitness by providing female Bewick’s Wrens (Thryomanes bewickii) and House Wrens (Troglodytes aedon) with food supplements. Males of these species vary in their rates of feeding; Bewick’s Wrens feed their incubating mates frequently, whereas House Wrens seldom engage in this behavior. Average length of incubation bout and nest attentiveness (proportion of time spent on the nest) were higher for supplemented female Bewick’s Wrens and House Wrens compared to controls. Furthermore, mates of supplemented Bewick’s Wrens provisioned females at lower rates than controls, and their rate of feeding was inversely correlated with ambient temperature. Incubation length and hatching success were not significantly different between treatments for either species. These results suggest that incubation mate feeding can increase female nest attentiveness and perhaps enhance fitness of both males and females. In House Wrens, potential tradeoffs between the benefits of parental care and opportunities to obtain additional mates may explain why males rarely feed incubating females. Received 1 July 2003, accepted 15 March 2004. In species that exhibit parental care, there is often a division of labor between sexes, with one sex primarily attending the nest. Consequently, trade-offs between offspring development and survival versus parental condition can exist if nest attentiveness is constrained by parental food limitation (Royama 1966). Food brought to the attending adult by the nonattending mate may ameliorate food limitation, and thus, offset these trade-offs (Smith et al. 1989). Feeding of incubating females by mates occurs in more than 40% of North American passerines (Kendeigh 1952) and is most pronounced in cavity nesters. Nonetheless, considerable variation in the rate of incubation mate feeding exists (Martin and Ghalambor 1999). Traditionally, incubation feeding was thought to maintain the pair bond between mates (Lack 1940, Kluyver 1950, Andrew 1961) or represent a premature attempt by males to feed nestlings (Skutch 1953, Nolan 1 Div. of Biology, Kansas State Univ., Manhattan, KS 66506, USA. 2 USGS-BRD Kansas Coop. Wildlife Research Unit, Div. of Biology, 204 Leasure Hall, Kansas State Univ., Manhattan, KS 66506, USA. 3 Current address: Dept. of Wildlife and Fisheries, Box 9690 Thompson Hall, Mississippi State Univ., Mississippi State, MS 39762, USA. 4 Current address: Dept. of Zoology, Weber State Univ., 2505 University Cir., Ogden, UT 84408, USA. 5 Corresponding author; e-mail: [email protected] 1958, Ricklefs 1974, Johnson and Kermott 1992). Both of these hypotheses have been challenged, and it has been suggested that food delivered to females constitutes an essential nutritional contribution (i.e., the food limitation hypothesis; von Haartman 1958, Royama 1966, Krebs 1970, Smith 1980, Nilsson and Smith 1988). Experimental tests of the potential adaptive benefit of incubation feeding (in terms of female attentiveness and hatching success) are relatively rare (e.g., Nilsson and Smith 1988, Moreno 1989, Smith et al. 1989). In this study, we examined effects of food supplements on female incubation behavior and correlates of fitness in two sympatric, secondary cavity-nesting species, Bewick’s Wren (Thryomanes bewickii) and House Wren (Troglodytes aedon). Incubation mate feeding is common in Bewick’s Wrens (Miller 1941), whereas male House Wrens rarely feed their mates during incubation (Johnson and Kermott 1992). We increased food available to incubating females by providing food supplements inside nest boxes (Nilsson and Smith 1988, Smith et al. 1989). This allowed females to have sole access to food without leaving nest cavities, simulating incubation feeding. If mate feeding constitutes an important contribution to females, we predicted that food supplements would enhance nest attentiveness. If additional food enhances female attentiveness, hatching success should increase and duration 24 THE WILSON BULLETIN • Vol. 116, No. 1, March 2004 of incubation should be reduced at supplemented nests relative to controls. If males do monitor female attentiveness, we also predicted they would adjust their rates of mate feeding accordingly and reduce their rate of incubation feeding to highly attentive females provided with food supplements (Smith et al. 1989).